Can't assign multi-field to MasterFields Property

Using D6,MS-SQL 2000,BDE and two TBDEClientDataSet components.  For the
detail dataset, I can't set multi fields,for example "Wno;Mno", to
MasetFields Property at design time. The IDE would raise an exception :
"masterdataset : Field "Wno;Mno" not found. " If I assign signle field , all
is fine. Why ?
Thanks.
Peter Peng